Why does retinal photoisomerization produce a nerve signal in the eye?

Retinal photoisomerization initiates a nerve signal in the eye through a complex biochemical cascade that translates light energy into electrical activity.

When a photon is absorbed by rhodopsin—a protein-pigment complex found in the retina—the retinal chromophore undergoes photoisomerization, changing its shape from 11-cis to all-trans. This structural alteration causes the retinal to shift its position within the protein, which in turn forces the opsin protein to change its own shape.123

This conformational change in the protein activates a G-protein called transducin, which triggers an enzyme to reduce levels of the second messenger cGMP. In the dark, cGMP keeps sodium channels open; however, the light-induced reduction of cGMP causes these channels to close. This leads to the hyperpolarization of the photoreceptor cell, which inhibits the release of neurotransmitters. This change in neurotransmitter release is then detected by intermediate neurons, ultimately generating the electrical impulses sent along the optic nerve to the brain.24